The present invention relates to a resin removing technique in a method for manufacturing a semiconductor device (or a semiconductor integrated circuit device).
In a semiconductor device wherein a semiconductor chip is mounted on a lead frame and is sealed together with the lead frame by resin molding, dam bars (tie bars) are each for coupling between leads of the lead frame and play the role of damming up molding resin flowing out between leads at the time of integrally sealing the lead frame and the semiconductor chip in the resin molding process.
The dam bars also play the role of reinforcing the leads. After sealing with the molding resin is over, the dam bars are cut off using, for example, a punch (cutting edge) and a die which are set in the interior of a press die (cutting die), whereby the leads of the lead frame are cut into individual leads from the state of being coupled together with the dam bars.
Japanese Unexamined Patent Publication No. Hei 4 (1992)-157761 (Patent Document 1) discloses, as a conventional tie bar cutting method, a technique wherein, using a punch and a die fitted precisely with the punch, which are set in the interior of a press die, a resin dam portion formed between a lead of a lead frame projecting from molding resin and another lead, as well as tie bar portion, are punched at a time. This publication also discloses a technique wherein a laser beam is radiated to the resin dam portion, allowing the resin of the dam portion to be melted and removed with the resulting heat, and thereafter the tie bar portion is cut off using the punch.
Japanese Unexamined Patent Publication No. Hei 8 (1996)-316396 (Patent Document 2) discloses a technique as a conventional method for manufacturing a resin-sealed semiconductor device wherein, using a lead frame having dam bars, intra-dam resin body formed between leads extending from a resin-sealed portion up to a dam bar, the intra-dam resin body having a thickness almost equal to that of each lead, is cut and removed together with the dam bar by means of a laser beam having a fine wedge-like spot.
According to a method for manufacturing a semiconductor device (or a semiconductor integrated circuit device) using a lead frame, in a process (resin sealing process, molding process) for forming a sealing body to seal a mounted semiconductor chip, there often is used a lead frame formed with dam bars (tie bars) as shown in Patent Documents 1 and 2 to prevent supplied resin from leaking to the outside of a device area on the lead frame.
After the resin sealing process, therefore, as shown in FIG. 1(a) and FIG. 3(a) of Patent Document 1, there is formed not only the main sealing body (chip sealing resin) for sealing the semiconductor chip but also a secondary sealing body (intra-dam resin, resin between leads) which is positioned around the semiconductor chip main sealing body in the area surrounded with leads exposed (projecting) from side faces of the main sealing body and dam bars.
In a dam cutting process after the resin sealing process, the dam bars and the intra-dam resin are removed and in a subsequent plating process a plating film as exterior plating is formed on each lead surface.
As a result of a study made by the present inventors about the method of removing the intra-dam resin in the dam cutting process, it turned out that the following problems were caused by the intra-dam resin removing method using a die punch (cutting edge) and by the methods disclosed in Patent Documents 1 and 2.
In case of removing a dam bar and intra-dam resin by punching with a die punch (cutting edge), the intra-dam resin can be removed together with the dam bar, as shown in FIG. 3(b) of Patent Document 1. However, as shown in FIG. 3(c) of Patent Document 1, a predetermined clearance taking a material tolerance and a cutting tolerance into account is provided between the area to be punched with the die punch and the leads adjacent thereto and the intra-dam resin remains partially on side faces of the leads, which is attributable to the presence of such clearance.
Moreover, in case of removing a dam bar and the intra-dam resin by radiation of a laser beam in accordance with the method disclosed in Patent Document 2, as shown in FIG. 23, resin residues 2i as residues of the intra-dam resin are formed on side faces of the associated lead 3 projecting from a side face 2c of a main sealing body 2. FIG. 23 is a plan view of a principal portion of a semiconductor device including a lead frame with a semiconductor chip mounted thereon, the lead frame being partially sealed with resin. FIG. 23 shows an area in which plural leads 3 as a portion of the lead frame are exposed partially from the side face 2c of the main sealing body 2.
Thus, with the resin residues 2i formed on side faces of each lead 3, there may occur the following inconveniences.
First, in a plating process for forming a plating film as exterior plating on surfaces of the leads 3 after the tie bar and intra-bar resin removing process, a plating solution no longer contacts the surfaces of the leads 3 due to intervention of the resin residues 2i, so that a plating film is not formed on the surfaces of the leads 3 covered with the resin residues 2i. Further, if the resin residues 2i fall off from the surfaces of the leads 3, the resin-removed portions are apt to undergo oxidation and corrosion because those portions are not formed with the plating film. As a result, when mounting the semiconductor device onto a package substrate by soldering, there occurs the problem that the leads 3 are not wetted by the solder.
In case of subjecting the leads 3 to a forming work after completion of the plating process, if lead forming is carried out with the resin residues 2i adhered to the surfaces of the leads 3 without falling off, a resin residue 2i, due to a forming stress, may peel off from the surface (a side face) of a lead 3 and face toward another lead 3 adjacent to the lead 3 with which the resin residue was in contact. At this time, since the resin residue 2i is scarcely in close contact with the plating film, the plating film peels off from the environs of the resin residue 2i, becomes an acicular (whisker-like, fine line-like) metal burr and faces toward another lead 3 adjacent to the lead 3 with which it was in contact, like the resin residue 2i. If the metal burr of the plating film is longer than the distance between adjacent leads 3 and comes into contact with the lead 3 adjacent thereto or if metal burrs project from two leads 3 so as to face each other and come into contact with each other, this state leads to the problem of an electrical shorting (short-circuit) between the leads 3.
There also is a case where the resin residues 2i formed on side faces of the leads 3 shown in FIG. 23 peel off from the lead side faces and, in a dam cutting process for removing dam bars (not shown) and intra-dam resin (not shown) with use of a die punch, the resin residues 2i thus peeled off drop into a die which is for removing those dam bars and intra-dam resin. Likewise, there is a case where a resin residue 2i drops into a die which is for bending a lead 3 into a desired shape. In these cases, the dropped resin residue 2i may bite into the lead 3 and flaw the lead surface or cause damage to the punch or die set in the press die.
Moreover, if an attempt is made to remove the intra-dam resin by radiation of a laser beam having such high energy as permits removal of the intra-dam resin 2f shown in FIG. 25 in a single laser beam radiation, in accordance with the method disclosed in Patent Document 2, there arises the problem that the resin melted by the heat of the laser beam scatters around as a foreign matter and adheres to the surfaces of the leads 3 exposed from the main sealing body 2. This foreign matter is very small like soot, so even if the laser beam used in removing the intra-dam resin 2f is radiated to the surface of a lead 3, not only is it difficult to remove the foreign matter, but also there is a fear that the foreign matter may adhere as burnt deposit onto the surface of the lead 3. If the foreign matter adheres to the surface of the lead 3, a plating film is not stably formed on the surface of the lead 3 in the subsequent plating process, so that such reliability elements as wettability with solder and adhesion to the leads are deteriorated. There also is the problem that if a laser beam having such high energy is radiated, the lead frame surface is for example oxidized due to a high temperature, causing a change in quality of the lead frame, with consequent failure to form a plating film on the surface of each lead 3 in the subsequent plating process.
Moreover, in the method disclosed in Patent Document 1, a laser beam is radiated to the whole area of each dam portion 12a shown in FIG. 25, so if an attempt is made to remove all the resin present within the dam portion 12a without leaving any resin therein, high energy is required for the radiation of a laser beam. In this case, like the problem of the method disclosed in Patent Document 2, a plating film is no longer formed on the surface of each lead 3 because a foreign matter created upon laser beam radiation adheres to the lead 3 surface. Even if the radiation of a laser beam is performed in multiple bursts as is disclosed in Patent Document 1, it eventually provides energy high enough to remove all the intra-dam resin 2f, thus giving rise to the same problem as above.
The larger the quantity of the resin residues 2i formed on the surfaces of the leads 3, the easier the occurrence of short-circuit between adjacent leads 3. A description will be given below about in what mechanism the resin residues 2i formed on the surfaces of the leads 3 remain in a large quantity.
Usually, as shown in FIG. 24, in the case where a copper plate 30, which is set over a die 32 within a press die and under a die punch guide 33, is to be punched with a die punch (cutting edge) 31 to form a lead frame of the copper plate 30, an sacrificial copper plate portion 30d is punched with the punch in a direction from an upper surface 30a toward a lower surface 30b of the copper plate 30 (in the direction of arrow shown in FIG. 24). At this time, as a result of punching with the die punch 31, as shown in FIG. 24, a corner of the copper plate 30 which is a boundary between the upper surface 30a and a side face 30c of the copper plate 30 is pushed by the die punch 31 and is pulled in toward the lower surface 30b of the copper plate 30 by the sacrificial copper plate portion 30d being punched. Consequently, the height of the copper plate 30d becomes lower than that of the upper surface 30a as a flat surface of the copper plate 30, thus creating a shape having curvilinear depressions (recesses) 30e. On the other hand, a corner of the copper plate 30 which is a boundary between the lower surface 30b and a side face 30c of the copper plate 30 is pulled in toward the lower surface 30b of the copper plate 30 by the sacrificial copper plate portion 30d being punched with the die punch 31, thus creating a burr 30f projecting downwards with respect to the lower surface 30b as a flat surface of the copper plate 30.
As described above, the lead frame formed by the die punch 31 has the projecting burrs 30f at lower surface ends thereof. Usually, the lower surface having the burrs 30f is turned upwards and is used as a main surface of the lead frame, with a semiconductor chip being mounted on the main surface. The reason is that if the lead frame formed with the downward burrs 30f is used as is, then when the semiconductor device is mounted onto a package substrate, a part or the whole of the lead frame floats due to the burrs 30f, with consequent fear of causing a packaging defect of the semiconductor device.
Thus, the lead frame with a semiconductor chip mounted thereon has the burrs 30f on its main surface side and has the curvilinear depressions 30e at back-side corners. This is also true of the leads which are a portion of the lead frame. Main surface-side corners of the leads are formed with the burrs 30f and their back surface-side corners are formed with the curvilinear depressions 30e. 
FIG. 25 is a plan view of the semiconductor device, showing a state wherein after mounting a semiconductor chip (not shown) onto the main surface of the lead frame and coupling the leads 3 with electrodes (not shown) on the semiconductor chip main surface electrically through bonding wires (not shown), the lead frame and the semiconductor chip were sealed with resin. Plural leads 3 exposed to the exterior from the main sealing body 2 are coupled together through dam bars 12. Within a dam portion 12a bordered by a dam bar 12, adjacent leads 3 and the main sealing body 2, intra-dam resin 2f is formed of the same material as the main sealing body 2.
FIG. 26 is a sectional view taken along line H-H in FIG. 25. As shown in FIG. 26, each lead 3 has burrs 30f on a main surface thereof and has depressions 30e on a back surface hereof. The intra-dam resin 2f located between adjacent leads 3 is formed in a state of getting in under the depressions 30e which have a curvilinear shape. This is because the surface of the die for forming the main sealing body 2 and the intra-dam resin 2f, which surface comes into contact with the back surface of the lead 3 having the depressions 30e, is flat, and when molding resin is poured into the die, it is intended for the molding resin to get into the clearance between the die having a flat surface and a lower surface of each depression 30e on the lead back surface to form the intra-dam resin 2f. Therefore, when a principal portion of the semiconductor device in FIG. 26 is seen from the back surface side, as shown in FIG. 27, the intra-dam resin 2f is formed on the surface of each back-side region of the lead 3 where the depression 30e (not shown) is formed. The intra-dam resin 2f is formed spreadedly to the lead 3 side or the dam bar 12 side with respect to the boundaries between side faces of each lead 3, as well as a side face of the associated dam bar 12, the side faces being indicated with broken lines in FIG. 27, and the intra-dam resin 2f. The intra-dam resin 2f formed on the back surface of each lead 3 covers the lead back surface in a wider range in the vicinity of the side face 2c of the main sealing body 2 than on the dam bar 12 side. In the direction along the side face 2c of the main sealing body 2, the back surface of each lead 3 is covered with the intra-dam resin 2f in a maximum range of about 100 μm (about 0.1 mm) from each of both ends. The width of each lead 3 in the direction along the side face 2c of the main sealing body 2 is about 300 μm (about 0.3 mm), so in the case where both ends of the back surface of the lead 3 are covered each about 100 μm with the intra-dam resin 2f in that direction, the width of the exposed lead 3 back surface is about 100 μm.
A lead frame (etching frame) 10d formed, not by punching with such a die punch 31 as shown in FIG. 24, but by exposing upper and lower surfaces of a copper plate with a resist film mask disposed thereon to a chemical has, in a sectional shape of a lead, projections 3i on a side face between the upper and lower surfaces of the lead frame as shown in FIG. 28. Therefore, in case of manufacturing the semiconductor device by using a lead frame 10d formed by etching, like a lead frame 10 formed by the die punch 31, in the resin sealing process of the lead frame 10d, intra-dam resin 2f is formed also above and below the projections 3i formed on a side face of each lead 3h. 
In the case where the intra-dam resin 2f is removed by the intra-dam resin removing method using a die punch or by radiating a laser beam from the main surface side of leads 3 shown in FIG. 26 in accordance with the method disclosed in Patent Documents 1 and 2, even if the intra-dam resin 2f on a side face of the associated dam bar 12 and side faces of leads 3h is removed, the intra-dam resin 2f formed below the depressions 30e on the back surfaces of the leads 3 cannot be removed completely because the die punch or the laser beam does not come into contact with the intra-dam resin 2f formed below the depressions 30e on the leads' back surfaces.
Also in case of using the aforesaid etching frame as the lead frame, the laser beam or the die punch 31 from the main surface side does not come into contact with the intra-dam resin 2f formed below the projections 3i on side faces of the leads 3 shown in FIG. 28, thus making it impossible to remove the intra-dam resin 2f located below the projections 3i. 
As described above, when a portion of the intra-dam resin 2f formed on side faces of the leads 3 or formed in lower portions of the leads 3 is not removed and resin residues 2i remain on the lead faces, the amount of resin formed on the lead faces becomes larger than in the case where each lead 3 is not formed with the depressions 30e and the side faces thereof are in contact perpendicularly with the main and back surfaces of the lead 3, with the side faces of the lead 3 being wholly perpendicular to the lead main surface. That is, in the case where the lead 3 is formed with the depressions 30e, it becomes easier for short-circuit to occur between adjacent leads 3, thus giving rise to a serious problem.